AudioService: Use device specific mode to open SCO audio

Bluetooth carkit may need specific mode to open SCO
channel. Add a settings value using BT device's MAC address
to store the SCO mode and use it to open SCO. In case no value
is stored, use default mode: Virtual Voice Call.

Change-Id: Iea31a48be0e8f3bd2d8689635fd8ce6f6d6da15a